In the related art, a method of welding two or more plates by friction stir welding to form a workpiece and rotating the workpiece to form a dome having a semispherical body shape is known (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-181576).In this method, before the friction stir welding is performed, a material, such as an aluminum scandium alloy, which delays growth of grains, is disposed between abutting edges between the plates.
Additionally, as a method of suppressing abnormal growth of grains accompanying solution heat treatment in a friction stir joint of an aluminum alloy, a method of performing intermediate anneal heat treatment after friction stir welding and performing solution heat treatment after the intermediate anneal heat treatment is known (for example, refer to Specification of U.S. Patent Application Publication No. 2012/0090738).